On an optical disk, tracks for writing information are arranged in a concentric circular form or in a spiral form on the recording surface. When information is to be written to or read from such an optical disk, a beam focused by an objective lens is radiated onto the disk in spot form and the desired tracks are traced. Since an objective lens and its support are heavy, a two-stage configuration is employed to enable the actuator to move a beam spot at high speed. A coarse actuator moves an objective lens and thus a beam spot roughly, while a fine actuator controls the position of the beam spot on the recording medium precisely. Conventional track access techniques are explained below with reference to pertinent documents.
PUPA No. 62-51037 discloses a technique for moving a beam spot mainly by means of a fine actuator. That is, a fine actuator is first driven so that the velocity of the beam spot relative to a disk matches a reference velocity. Then, a coarse actuator is driven by a signal indicating the displacement of the beam spot that is caused by the fine actuator, and this displacement is compensated for. Since the driving of the coarse actuator is delayed, it is difficult to make maximum use of its acceleration and deceleration capability. Moreover, at the time of acceleration and deceleration, the beam spot is displaced. Since the amount of displacement is proportional to the acceleration and deceleration, the beam spot is widely displaced when the acceleration or deceleration is great. The impossibility of obtaining reliable tracking error signals (hereafter referred to as TES) causes problems. Furthermore, because of this large displacement, there is a problem in that a large transient shock is transmitted from the coarse actuator to the fine actuator and the beam spot leaves the desired track when an attempt is made to hold the position of the beam spot in the center of tracks in order to write or read information after track access.
PUPA No. 1-290176 discloses a technique for transmitting to a coarse actuator a signal representing the difference between the drive signal (reference velocity signal) and the travel velocity signal of a beam spot, and for transmitting a high-frequency component of the signal to a fine actuator. In general, a coarse actuator cannot respond quickly, because its moving parts are heavier than those of a fine actuator. Therefore, the unit is designed to supply a quick response component to a fine actuator so that an objective lens can move over a disk surface correctly. However, this technique has problems in that the beam does not pass through the center of the objective lens when the objective lens slides slowly due to the movement of the coarse actuator, especially since, for an axial rotating-type fine actuator, it is difficult to manufacture an optical head so that the rotation shaft strictly matches the center of gravity of the fine actuator. Therefore, in reality, an objective lens driven by a fine actuator is accelerated proportionally to the acceleration of the coarse actuator. As a result, it slides slowly. However, this kind of movement cannot be compensated for by the high-frequency component of the above difference signal. If the objective lens is not positioned at the center of the beam by means of a spring when the fine actuator is not being driven, the beam cannot be controlled so that it passes through the center of the objective lens unless a direct current component is added to the difference signal.
In the conventional technology, when the velocity of a beam spot relative to an optical disk is to be controlled, a reference velocity generally recorded in ROM is read out according to the distance (number of tracks) from the current position of the beam spot to a target track. The reference velocity can be determined precisely according to the number of tracks, so that the velocity will be zero at the beginning and end of track access, and greatest in the middle. However, increasing the ROM capacity is problematic because a large amount of information must be stored in ROM. Neither of the aforementioned references shows an effective solution to this problem.